Conventionally, there is an image forming apparatus such as a multi-function peripheral (hereinafter, referred to as an “MFP”) and a printer. The image forming apparatus is equipped with a fixing apparatus. The fixing apparatus heats a conductive layer of a belt with an electromagnetic induction heating system (hereinafter, referred to as an “IH system”). The fixing apparatus fixes a toner image on an image receiving medium through the heat of the belt. The conductive layer of the belt generates heat via application of an induction current. In order to shorten the warming-up time, the fixing apparatus reduces the heat capacity of the belt. In order to replenish insufficient calorific value of the belt, the fixing apparatus is equipped with a magnetic material. The magnetic material enables a magnetic flux generated at the time of the electromagnetic induction heating to be concentrated in order to increase the calorific value of the belt. For example, the magnetic material is a magnetic shunt alloy.
Generally, the fixing apparatus keeps the belt at a preset fixing temperature to maintain a fixable state at the time of forming an image. At least in a standby state in which no print request is received, in order to save electric power, the fixing apparatus keeps the belt at a standby temperature lower than the fixing temperature. The standby temperature is set in a range from a temperature at the time of non-heating to the fixing temperature. The standby temperature is set to a temperature at which the belt can be rapidly heated to the fixing temperature when the fixing apparatus changes from the standby state to a fixing operation. The heating of the belt is adjusted by an electric power control. In the standby state, in order to keep the temperature of the belt (hereinafter, referred to as “belt temperature”) constant, an induction current generation section is controlled to make output of the induction current constant.
Incidentally, in the standby state, an initial value of a frequency applied to the induction current generation section is determined by a target value of an output (hereinafter, referred to as “IH output”) of the induction current generation section. In a case in which the magnetic material is the magnetic shunt alloy, magnetism of the magnetic material sharply changes from ferromagnetism to paramagnetism if the temperature thereof exceeds a Curie point thereof. In a case in which the magnetic material is the magnetic shunt alloy, the magnetism of the magnetic material slowly changes from the ferromagnetism to the paramagnetism if the temperature thereof becomes high despite not exceeding the Curie point thereof. If the magnetism of the magnetic material changes, a load (hereinafter, referred to as an “IH load”) of the induction current generation section also changes. Through the change of the IH load, a proper initial value of a frequency changes. If the proper initial value of the frequency cannot be set, the IH output is deviated from the target value, and it is difficult to keep the belt temperature constant in the standby state. For example, if the IH output is excessively high, the belt temperature is excessively increased in the standby state, and thus there is a possibility that the belt is damaged. On the other hand, if the IH output is excessively low, the belt temperature cannot be sufficiently increased in the standby state, and there is a possibility that the belt cannot be kept at a proper standby temperature.